Visual audio frequency comparator

ABSTRACT

A system for determining the frequency relationship between two notes of a musical instrument includes a split screen display device, and bar generating means connected to the instrument and display device for generating a first plurality of bars representative of the frequency of the first note on one portion of the display device and a second plurality of bars representative of the frequency of the second note on another portion of the display device. The vertical sweep circuit of the display device is synchronized to the frequency of one of the notes.

United States Patent 91 Lester et al.

[ Apr.8, 1975 41 VISUAL AUDIO FREQUENCY COMPARATOR [75] Inventors:Theodore V. Lester, Schiller Park;

Donald C. Ryon, Carol Stream, both of III.

[73] Assignee: Motorola, Inc., Chicago, Ill.

[22] Filed: Sept. 17, 1973 211 Appl. No.: 398,027

521 u.s.c| 324/7911; 324/82; 324/88; 84/454 51 mu ..G0lr23/l4 [58] Fieldof Search... 324/78 R, 78 Q, 79 R, 82 X, 324/88, 121, 91; 84/454 [56]References Cited UNITED STATES PATENTS 3.074.054 l/l963 Ross 324/82 X3.287.639 ll/l966 Vail 324/88 X OTHER PUBLICATIONS Tolley, IBM Tech.Disc. Bul., Vol. 14, No. 4. Sept.

OSCI LLATORS AND DIVIDERS OSCILLATORS AND D IVIDERS PATTERN GENERATORl97l,p. 1266.

Primary Examiner-Alfred E. Smith Assistant ExaminerRolf I-IilleAttorney, Agent, or FirmEugene A. Parsons; Vincent J. Rauner 5 7ABSTRACT A system for determining the frequency relationship between twonotes of a musical instrument includes a split screen display device,and bar generating means connected to the instrument and display devicefor generating a first plurality of bars representative of the frequencyof the first note on one portion of the display device and a secondplurality of bars representative of the frequency of the second note onanother portion of the display device. The vertical sweep circuit of thedisplay device is synchronized to the frequency of one of the notes.

4 Claims, 6 Drawing Figures VERTICAL SYNC.

sumanrg SAME FREQUENCY AND PHASE SAME FREQUENCY-I80 OUT OF PHASE 2IlOCTAVE 514 A THIRD INTONE lNTERVAL VISUAL AUDIO FREQUENCY COMPARATORBACKGROUND 1. Field of Invention This invention relates generally tofrequency comparison systems, and more particularly to systems fordetermining the frequency relationship between two notes provided by amusical instrument.

There are many applications wherein it is necessary to compare thefrequency of one signal to another. One such application is in anelectronic musical instrument wherein it is necessary to compare thefrequencies of various notes sounded by the instrument for tuning,demonstration or experimentation purposes.

2. Prior Art Several techniques for determining the frequencies ofsignals are known. Such techniques include the use of tuned reeds whichvibrate when the frequency of the signal applied thereto is equal to theresonant frequency of the reed, beat note systems wherein the frequencyof the note is compared with the frequency of an oscillator having aknown frequency and visual comparison systems employing Lissajousfigures.

Whereas these techniques provide a way to determine frequencies ofsignals, the reed and standard oscillator systems are costly and onlyprovide an accurate frequency determination at discrete points and donot provide a direct frequency comparison between two signals. TheLissajous display systems provide a direct comparison, but the displaycannot be readily interpreted unless the ratio between the signalsapplied thereto is an integral number, and interpretation of Lissajousfigures becomes extremely difficult when frequencies of a musical scale,which have a non-integral frequency relationship therebetween, arecompared.

SUMMARY It is an object of the present invention to provide an improvedvisual frequency comparison circuit for comparing the frequency ofmusical notes.

It is a further object of the invention to provide a frequencycomparison circuit that readily provides a comparison between signalshaving a non-integral frequency relationship therebetween.

It is a further object of this invention to provide a visual frequencycomparison system that has a low cost and is readily interpreted bypersonnel untrained in electronics.

In accordance with a preferred embodiment of the invention, a splitscreen video monitor is employed. A bar generator is used to generate aplurality of bars indicative of the frequency of one of the notes in onearea of the split screen and a second plurality of bars indicative ofthe frequency of a second note in another portion of the split screen.The vertical sweep circuit of the video monitor is synchronized to thefrequency of one of the notes, thereby providing a stationary barpattern on one portion of the screen. The frequency of the signaldisplayed in the other portion of the screen is compared to the fixedpattern by comparing the relative number of bars and the position andmovement of the bars with respect to the fixed bar pattern.

In the drawingsf FIG. 1 is a block diagram of the visual frequencycomparison circuit according to the invention shown connected to anelectronic organ;

FIG. 2 is a diagram, partially in schematic form and partially in blockdiagram form, of the visual display device according to the invention;and

FIGS. 3-6 are drawings showing the bar patterns obtained for variousfrequency ratios of signals applied thereto.

DETAILED DESCRIPTION Referring to greater detail to FIG. 1, there isshown a portion of a musical instrument including oscillators anddividers 10, 20 and keyboards 12, 22 connected, respectively, thereto. Apair of amplifiers 14, 24 have inputs thereof connected to the keyboards12,22 and outputs connected to a pattern generator 16. The patterngenerator 16 is connected to display means such as a video monitor 18and generates bars in first and second portions of a split screen 26 and28. Vertical synchronization for the monitor 18 is applied thereto fromthe pattern generator 16, and horizontal synchronization is applied fromthe monitor 18 to the pattern generator 16.

In operation, the keyboard 12 passes a first tone from the oscillatorsand dividers 10 to the input of the amplifier 14. The amplified signalfrom amplifier 14 is processed by the pattern generator 16 and a videosignal which is substantially a square or rectangular wave having thesame frequency as the frequency of the first tone is applied to themonitor 18 to generate the bar pattern in the area 26. Similarly, asignal from the oscillators and dividers 20 is passed by the keyboard 22to the amplifier 24. The pattern generator 16 generates a video signalin response to the signal from the amplifier 24 to generate the barpattern in the area 28. A vertical synchronization signal is applied tothe monitor 18 to synchronize the vertical sweep of the monitor to asubmultiple of the frequency of the signal from the amplifier 14. Ahorizontal synchronization signal is obtained from the horizontalsynchronization circuitry of the monitor 18 and controls a gate circuitin the pattern generator 16 to alternately pass signals from theamplifiers l4 and 24 to the video portion of the monitor 18 to generatethe patterns appearing in areas 26 and 28. A more detailed explanationof the operation of the pattern generator 16 and the monitor 18 follows.

Referring to FIG. 2, there is shown a schematic diagram of oneembodiment of the pattern generator 16 and a more detailed block diagramof a typical monitor 18. The pattern generator 16 has an input point 34which is connected to the output of theamplifier 14. The input point 34is also connected to the base of a transistor 36 through a capacitor 38and a resistor 40. Similarly, an input point 44 is connected to theoutput of the amplifier 24 and to the base of a transistor 46 through acapacitor 48 and a resistor 50. The collectors of the transistors 36 and46 are connected to a power supply A+ through resistors 42 and 52,respectively, and to the input of a video amplifier in the monitor 18through a pair of resistors 63 and 64, respectively. The collector ofthe transistor 36 is also connected to the vertical oscillator 54. Apair of transistors 62 and 72 have the collectors and emitters thereofconnected to the collectors and emitters of the transistors 36 and 46,respectively, and the emitters of the transistors 36, 46, 62 and 72 areconnected to a ground or common potential. The base of the transistor 62is connected by means of a resistor 68 to the collectors of a pair oftransistors 64, 66, each having an emitter connected to ground.Similarly, the base of the transistor 72 is connected through a resistor78 to the collectors of a pair of transistors 74 and 76, whose emittersare also connected to ground. The collectors of the transistors 74 and76 are connected to the power supply A+ through a resistor 79. Thecollectors of the transistors 64 and 66 are also connected to the powersupply A+ through a resistor 69 and to the bases of the transistors 74and 76 through resistors 80 and 82, respectively. The bases of thetransistors 64 and 66 are connected to the output of a horizontaloscillator 90 in the monitor 18 by means of resistors 84 and 86,respectively, and a capacitor 88. Resistor 87 is adjusted to split thescreen in the exact center of the raster.

The output of the horizontal oscillator 90 is also connected to ahorizontal output circuit 92 which drives the horizontal sweep coils ofa yoke 94 and a high voltage circuit 96 which is connected to the anodeof a display device such as a cathode ray tube 98. A vertical outputcircuit 100 of the monitor 18 has an input connected to the output ofthe. vertical oscillator 54 and an output connected to the verticalsweep windings of the yoke 94. Thea-output of the video amplifier 60 isconnected to the grid 102 of the cathode ray tube 98 to modulate theraster produced on the face of the cathode ray tube 98 by the horizontaloutput circuit 92 and the vertical output circuit 100.

In operation, a tone signal applied to the input point 34 is amplifiedand amplitude limited by the transistor 36 to provide a substantiallyrectangular wave to the input of the amplifier 60 when the transistor 62is rendered nonconductive. Similarly the transistor 46 amplifies signalsfrom the input point 44 for application to the amplifier 60 when thetransistor 72 is nonconductive. The transistors 62 and 72 serve to shuntthe signals from the respective transistors 36 and 46 to ground wheneither of the transistors 62 and 72 is conductive.

The horizontal oscillator 90 provides oscillations at a relatively highhorizontal sweep frequency which may be 15,750 Hz or other suitablefrequency to the horizontal output circuit 92 which provides a rampfunction to the horizontal coils of the yoke 94 to horizontally sweepthe beam across the face of the cathode ray tube 98. The flyback pulsesfrom the horizontal output stage 92 are, transformed to a high voltageand rectified by the high voltage circuit 98 to provide the anodevoltage for the cathode ray tube 98 in a conventional fashion. Theoutput of the horizontal oscillator 90 is also coupled through thecapacitor 88 to the bases of the transistors 64 and 66 which areeffectively connected in parallel. A single transistor may be used inplace of the transistors 64 and 66, however because integrated circuitcomprising four dual input gates was used in the fabrication of thepattern generator 16, one of the gates consisting of transistors 64 and66 was connected as an amplifier as shown.

Because the output of the horizontal oscillator 90 is capacitivelycoupled to the transistors 64 and 66, both transistors are renderedconductive when the value of the alternating current component of thesignal from the horizontal oscillator 90 is sufficient to forward biasboth transistors. When the value of the alternating current component isless than the value required to forward bias the base-emitter junctionsof the transistors 64 and 66, both transistors 64 and 66 are renderednonconductive. The output at the collectors of the transistors 64 and 66is therefore substantially a square or rectangular wave having afrequency equal to the frequency of the signal generated by thehorizontal oscillator 90.

The square wave signal from the transistors 64 and 66 is applied to thebase of the transistor 62 to render the transistor 62 conductive whenthe square wave is at its high state and to render the transistor 62nonconductive when the square wave is in its low state. The

signal from the transistors 64 and 66 is also applied to the bases ofthe transistors 74 and 76 which have a similar configuration to thetransistor pair 64, 66, and which serve as a phase inverter to generatea second wave 180 out of phase with the square wave applied thereto. Thesignal from the collectors of the transistors 74 and 76 is applied tothe base of the transistor 72 to render the transistor 72 alternatelyconductive and non-conductive, transistor 72 being conductive whentransistor 62 is non-conductive and vise versa.

Since the transistors 72 and 62 are alternately rendered conductive, oneof the signals from the transistors 46 and 36 is alternately shunted toground while the signal from the other of the transistors 46 and 36 isapplied to the video amplifier 60. The aforementioned action causes thebeam of the cathode ray tube 98 to be modulated by the video amplifier60 in response to the signal from the transistor 36 during the firstportion of each horizontal sweep, and in response to the signal from thetransistor 46 during the remainder of the sweep, thereby generating thesplit screen effeet.

The signal from the transistor 36 is also applied to the verticaloscillator 54 to provide a synchronization pulse thereto which causesthe vertical oscillator to operate at a frquency which is a submultipleof the frequency of the signal from the transistor 36. The verticaloscillator 54, which generally operates at a lower frequency than thefrequency of the horizontal oscillator 90, causes the vertical outputcircuit to apply a ramp signal to the vertical winding of the yoke 94 tocause the beam of the cathode ray tube 98 to be vertically swept at asubmultiple of the frequency of one of the tones applied to thefrequency comparator. The synchronization causes the bar displaycorresponding to the tone to which the system is synchronized to appearas a series of stationary horizontal bars, the number of bars beingdetermined by the frequency ratio of the tone signal and the operatingfrequency of the vertical oscillator 54. The number of bars appearing inthe display area for the other of the signals is determined by thefrequency ratio between that signal and the frequency of the verticaloscillator 54 and serves to provide a frequency comparison between thetwo signals applied to the comparator according to the invention.

Referring to FIG. 3, the bar pattern generated in the areas 26 and 28 ofthe monitor 18 indicates that the frequencies being compared by thevisual frequency comparison system according to the invention have boththe same frequency and phase because the number of bars in both theregions 26 and 28 is equal and the bars lie in the same horizontal line.

The pattern shown' in FIG. 4 indicates that the two signals have thesame frequency since the spacing between the bars in each of the areas26 and 28 is equal,

in the area 26. Similarly, FIG. 6 shows a 5:4 ratio, five bars beingpresent in area 28 for every four present in the area 26.

The display provided by the system according to the invention is alsouseful for comparing signals having only a slight frequency offsettherebetween. Since the vertical sweep of the monitor 18 is synchronizedto the signal displayed in the area 26, the bars in the area 26 willremain stationary. The bars in the area 28 will remain stationary whenthe frequency of the signal displayed in area 28 is related to thefrequency of the signal displayed in area 26 by a ratio of small wholenumbers such as, for example, 1:1, 2:1 or 5:4 as shown in FIGS. 3-6. Ifthe aforementioned ratio does not occur, the bars in the area 28 willroll upward or downward with respect to the bars in the area 26. Anupward movement shows that the signal displayed in area 28 has aslightly higher frequency, or a sharp relationship to the signaldisplayed in area 26, while a downward movement shows a lower or flatpitch.

For example, an upward movement of the bars in area 28 of FIG. 3 showsthat the frequency of the signal displayed in area 28 is slightly higherthan the frequency of the signal displayed in area 26. Similarly, anupward movement of the bars in area 28 of FIG. 5 shows that thefrequency ratio is slightly greater than 2:l while an upward movement inthe area 28 of FIG. 6 indicates a slightly greater ratio than 5:4. Thus,the frequency relationship between the two signals may be readilyascertained by counting the ratio of the bars displayed in area 26 and28 and noting whether the bars in area 28 move upward or downward withrespect to the bars in area 26.

The inventors have found that the system according to the inventionprovides an effective and accurate way to make frequency comparisonsparticularly of the type of comparisons necessary for tuning musicalinstruments.

Whereas the embodiment of the invention described in the foregoing showsapparatus for generating a plurality of horizontal bars, it should beobvious to those skilled in the art that vertical bars can also begenerated by simply rotating the display tube by 90, by causing thehorizontal rather than the vertical oscillator to operate at asubmultiple frequency of one of the tone frequencies applied thereto, orby other means. Also, another display device such as a solid statedisplay device may be used in place of the cathode ray tube 98. Itshould, however, be noted that these and other modifications readilyapparent to those skilled in the art still fall within the scope andspirit of the invention.

We claim:

1. A system for comparing the frequencies of first and second musicalnotes including in combination:

means for receiving a first alternating current signal having apredetermined frequency relationship to the frequency of said firstmusical note; means for receiving a second alternating current signalhaving a predetermined frequency relationship to the frequency of saidsecond musical note;

display means for visually displaying bars in response to alternatingcurrent signals applied thereto, the number and position of the barsdisplayed being proportional to the frequency and phase, respectively,of the alternating current signals applied to said display means saiddisplay means including a display device, and a high frequency sweepcircuit and a low frequency sweep circuit coupled to said displaydevice, one of said sweep circuits being horizontal and the other beingvertical for generating a raster on said display device; and

gating means connected to said first and second alternating currentsignal receiving means and to said display means for alternatelyapplying signals, representative of one of said first and secondalternating current signals, to said display means an output of saidhigh frequency sweep circuit being connected to said gating means forcausing said gating means to alternately apply one of said first andsecond alternating current representative signals to said display meansfor a portion of each high frequency sweep.

2. A system as recited in claim 1 wherein said low frequency sweepcircuit is connected to said first signal receiving means, said lowfrequency sweep circuit being responsive to said first signal receivingmeans for synchronizing the frequency of operation of said low frequencysweep circuit 0 a submultiple of the frequency of said first signal.

3. A system as recited in claim 2 wherein said first and second signalreceiving means include a limiting amplifier for generating signalshaving a substantially rectangular wave shape in response to said firstand second alternating current signals applied thereto.

4. A system as recited in claim 3 further including means for modulatingthe raster generated by said horizontal and vertical sweep circuits,said modulating means being connected to said gating means and to saiddisplay device for modulating the raster in response to said first andsecond alternating current signals.

1. A system for comparing the frequencies of first and second musicalnotes including in combination: means for receiving a first alternatingcurrent signal having a predetermined frequency relationship to thefrequency of said first musical note; means for receiving a secondalternating current signal having a predetermined frequency relationshipto the frequency of said second musical note; display means for visuallydisplaying bars in response to alternating current signals appliedthereto, the number and position of the bars displayed beingproportional to the frequency and phase, respectively, of thealternating current signals applied to said display means said displaymeans including a display device, and a high frequency sweep circuit anda low frequency sweep circuit coupled to said display device, one ofsaid sweep circuits being horizontal and the other being vertical forgenerating a raster on said display device; and gating means connectedto said first and second alternating current signal receiving means andto said display means for alternately applying signals, representativeof one of said first and second alternating current signals, to saiddisplay means an output of said high frequency sweep circuit beingconnected to said gating means for causing said gating means toalternately apply one of said first and second alternating currentrepresentative signals to said display means for a portion of each highfrequency sweep.
 2. A system as recited in claim 1 wherein said lowfrequency sweep circuit is connected to said first signal receivingmeans, said low frequency sweep circuit being responsive to said firstsignal receiving means for synchronizing the frequency of operation ofsaid low frequency sweep circuit o a submultiple of the frequency ofsaid first signal.
 3. A system as recited in claim 2 wherein said firstand second signal receiving means include a limiting amplifier forgenerating signals having a substantially rectangular wave shape inresponse to said first and second alternating current signals appliedthereto.
 4. A system as recited in claim 3 further including means formodulating the raster generated by said horizontal and vertical sweepcircuits, said modulating means being connected to said gating means andto said display device for modulating the raster in response to saidfirst and second alternating current signals.